Little attention has so far been accorded to the removal of zinc in connection with nickel production. On one hand, raw materials which contain only small concentrations of zinc have been used; furthermore, being easily volatile it has generally been removed to a sufficient degree during the pyrometallurgical treatment stages. On the other hand, because of this great demands have not been set regarding the zinc content in the nickel produced.
In recent years, however, increasing purity demands have been set on nickel products, also in regard to zinc. At the same time raw materials which contain increasing quantities of impurities have been taken into use. Furthermore, as the production of nickel in certain cases is performed entirely without the pyrometallurgical stage, zinc removal has drastically gained in importance.
Only a few processes for zinc removal are known in connection with the hydrometallurgical treatment of nickel. The most common method is to remove zinc in the form of a sulfide by using hydrogen sulfide. The disadvantages of this process are: poisonous nature of hydrogen sulfide and the difficulty of obtaining a sufficiently zinc-free solution without considerable nickel losses into the produced sulfide precipitate.
In principle, two types of processes have been introduced in patent literature for the extraction of zinc from nickel solutions. The first group comprises processes which require the presence of a certain anion, mainly chloride, iodide, thiocyanate, etc., in which case the zinc can be extracted by means of extraction agents of the anion-exchanger type, such as amines (U.S. Pat. Nos. 3,446,720 and 3,194,652) or sulfonium thiocyanate (British Pat. No. 1,314,924). The second group consists of processes in which the zinc is extracted by means of extraction agents of the cation-exchanger type, such as di-(2-ethyl-hexyl) phosphoric acid (U.S. Pat. Nos. 3,666,446 and 2,992,894) or salicylic aldoximes (German Offenlegungsschrift No. 2,334,901).
The object of the present invention is to provide a process for removing zinc by extraction in connection with the hydrometallurgical production of nickel. The nickel solution from which zinc is removed is usually prepared by leaching ore, concentrates, intermediate products obtained by pyro- or hydrometallurgical processes, or nickel-bearing scrap metal.
The object of the invention is specifically to provide a process by which the problems connected with the use of organic phosphoric acids in zinc extraction can be eliminated. Organic phosphoric acid here denotes compounds with the following general formula ##STR1## in which R.sub.1 and/or R.sub.2 are alkyl, aryl, or alkyl-aryl radicals, mutually either similar or different, in which case only one of them can be hydrogen. This extraction agent is dissolved in an organic solvent, usually a hydrocarbon or a hydrocarbon mixture, especially light petroleum (kerosene). This organic solution can also contain such additives as alcohols or tributhyl phosphate.
One of the greatest drawbacks of such extraction processes is that some of the organic solution is always left in the refined solution obtained, part of it actually dissolved in the obtained aqueous solution, and part of it in the form of small, unseparated microdroplets. This loss of organic material is a substantial cost factor, and in addition it may, specifically in nickel processes, affect the quality of the final product. It has been noticed that, if the last stage of nickel production consists of an electrolytic reduction of nickel, even small quantities of the extraction agent or of the hydrocarbon used as a solvent, worsen the appearance of the cathodes. When hydrogen reduction is used for the production of nickel, the said organic substances may increase the carbon contents in the nickel.
Several processes are currently in use for the removal of organic substances from the aqueous solutions of extraction. The quantity of the organic solution loss can be decreased with the aid of different coalescers or by froth flotation. The same can be achieved by using specially-constructed settling devices or a centrifuge for the separation of the different phases of the dispersion produced by extraction. Activated carbon filtration has been noted to be an effective remover or organic droplets. All these processes are, however, expensive in regard to both investment and operation. By the process according to the present invention the use of separate additional devices can be eliminated.